1. Field of the Invention
The invention relates in general to an electrochromic device and method of fabricating the same, and more particularly to the pressure sensitive electrochromic device and method of fabricating the same.
2. Description of the Related Art
In the recent thirty years, Electro Optical technology has paid attention to the electrochromic device. Electrochromic devices are devices in which a physical/chemical change produced in response to the induced electric field results in a change in the reflective or transmissive properties of the device with respect to electromagnetic radiations, e.g., UV, visible and IR radiations. Also, the electrochromic image formed by the application of an appropriate voltage to an electrochromic cell persists for a useful period after the activating voltage is discontinued, generally until it is erased by application of an appropriate voltage of reversed polarity. Simply saying, electrochromism is related to color change by the reversible electrochemical process. Due to the reversible electrochemical process, the electrochromic layer changes between the bleached state and the colored state.
The electrochromic layer usually comprises an inorganic metal oxide, most commonly a transition metal oxide, in particular tungsten oxide (WO3). The reversible electrochemical reaction of tungsten oxide:WO3+xM++xe−MxWO3 
M is hydrogen anions (H+), lithium anions (Li+) or sodium anions (Na+); x is typically up to about 0.5, determined by the electric current passing the electrochromic layer. Colorless or light yellow tungsten oxide will be reduced and becomes blue or deep blue tungsten bronze (MxWO3) due to the injection of M+ and e−.
As a electric current is applied, M+ and e− are conducted into tungsten oxide so as to reduce colorless tungsten oxide to blue tungsten bronze (MxWO3); this is known as “coloration”. If blue tungsten bronze loses M+ and e−, the colored electrochromic layer will be uncolored (by oxidizing blue tungsten bronze to colorless tungsten oxide); this is known as “bleaching reaction”.
The electrochromic layer may be selected from any electrochromic material, including cathodic electrochromic materials, anodic electrochromic materials and cathodic/anodic electrochromic materials; many of which are well known to those skilled in the art and commercially available. Cathodic electrochromic materials (i.e. cathodic coloration materials) include non-stoichiometric (i.e., oxygen deficient) metal oxides wherein the metal has variable oxidation states. Exemplary of such cathodic electrochromic materials are tungsten oxide, molybdenum oxide, vanadium oxide, titanium oxide, lead oxide, and bismuth oxide and compatible mixtures of any of them. Anodic electrochromic materials include fully oxidized compounds comprising metal wherein the metal has variable oxidation states. Exemplary of such anodic electrochromic materials are Prussian blue, iridium oxide and nickel hydroxide and compatible mixtures of any of them. The most commonly used and studied electrochromic material is tungsten oxide (WO3), due to its highest coloration efficiency, excellent reversibility, low price and non-toxicity.
The electrochromic device is an electrochemical system and generally includes several thin films. The must-have electrochromic layer comprises one, two or more than two electrochromic materials. FIG. 1 (prior art) is a cross-sectional view of a conventional electrochromic device. The conventional electrochromic device 1 comprises a first substrate 11 and a second substrate 12 disposed opposite; a first transparent conducting film 13 and a second transparent conducting film 14 respectively formed on the first substrate 11 and the second substrate 12; a cathode 15 and an anode 16; an electrolyte (solid or liquid state) 17 between the cathode 15 and the anode 16; an external circuit provided to the first transparent conducting film 13 and the second transparent conducting film 14 being connected to a voltage providing source. The first substrate 11 and the second substrate 12 could be made of the transparent glass. The first transparent conducting film 13 and the second transparent conducting film 14 could be made of conductive indium tin oxide (ITO). At least one of the cathode 15 and the anode 16 comprises the electrochromic material which is responsible for the major color change of the electrochromic device; the one is knows as “electrochromic working electrode”, and the other is knows as “counter electrode”.
If the counter electrode is substituted by a layer containing the other electrochromic material, which means the device comprises both anodic and cathodic electrochromic materials (i.e. electrochromic materials darkened or bleached simutaneously), the device is known as a “complementary electrochromic device”. The complementary electrochromic device has wider range of optical density, increased transmittance variation and more colors in change.
The electrochromic device has been widely applied to various applications. If the electrochromic device comprises every transparent layer, it can be used to construct the “smart windows” (a new generation of windows) for the building or the cars for the purpose of energy saving. Flip a switch and an electrochromic smart window can change from clear to fully darkened or any level of tint in-between. The changeable windows allow for privacy, to cut down glare, and to ward off increases in solar heat. The action of an electric field signals the change in the window's optical and thermal properties. Once the field is reversed, the process is also reversed. Many applications such as the sunglasses, the advertisement posters, the reflectance-adjustable automatic rearview mirrors (protecting the driver from glare effects), the current indicators and the light filter are also commercially available. Early research indicates that electrochromic technology can save substantial amounts of energy, and may eventually replace traditional solar control technology such as tints, reflective films and shading devices.